This invention relates generally to a system and method for calibrating a virtual robot tool center point (TCP) or virtual work-object frame and more particularly to the use of relative measurement to perform in-process workcell calibration.
Industry is now seeing a dramatic increase in robot simulation and off-line programming. In order to use off-line programming effectively, the simulated workcell has to be identical to the real workcell. This requires a more efficient and accurate method for robot calibration. By making use of calibration, the simulated robot workcell will clone the real workcell in a simulation model, so that the off-line generated robot program from a simulated workcell will be accurate enough and can be directly downloaded to a real robot controller to drive the real robot with maximum accuracy and without further modification.
A variety of attempts to develop a better robot calibration system and method to improve robot accuracy exist in the prior art. Currently used techniques, however, are typically tedious, time consuming and expensive. This is because most of the prior art calibration methodology so far is based on absolute calibration.
xe2x80x9cAbsolute calibrationxe2x80x9d refers to the method by which an external coordinate measurement system is utilized to measure the absolute position, often referred to as a global coordinate system. Since the external system measures the coordinates of a point in the workspace, the absolute method can validate any path accuracy. However, absolute position measurement has many drawbacks including the fact that it is time consuming, expensive and sometimes fails to meet accuracy requirements. One example in the prior art is to use an optical coordinate measurement system (OCMS) to calibrate the robotic workcell, which is a very expensive and time-consuming way of calibrating the robot.
In contrast to absolute calibration, some development has been made in the area of xe2x80x9crelative calibrationxe2x80x9d. Relative calibration is a method in which a standard reference target is used as the precision reference for the correction of robot kinematic error. This xe2x80x9cstandard referencexe2x80x9d provides high-precision relative geometric quantities such as length, circularity and linearity. A standard reference could simply be a bar, a cube, a cylinder, or a ball. During the calibration, the robot is driven to make the tool center point (TCP) follow the geometry of the selected standard reference. This standard reference therefore provides a constraint on the TCP process. Due to the kinematic error, this constraint would be violated if the nominal kinematic model were used to calculate the Cartesian coordinates from the same joint angles. Minimization of the constraint violation (constraint error) will give the values of error parameters. In the present invention, this standard is called xe2x80x9crelative reference.xe2x80x9d
However, all known relative calibration techniques are only for one component calibration. There is no relative calibration technique to deal with the overall workcell calibration. Accordingly there is a need for an economical calibration method and apparatus to deal with overall workcell calibration.
Moreover, the tool center point (TCP) may change due to tool wear or tool changes. The workpiece itself can introduce a significant amount of error or uncertainty due to workpiece variation or deflection during the manufacturing process. Real time calibration for each workpiece can eliminate this effect. Accordingly, there is a need to develop a method and apparatus, which must be cost effective and capable of in-process operation and real-time implementation.
In order to overcome the shortcomings and drawbacks of conventional calibration systems and methods to make calibration cost-effective, efficient and easy to use, the objective of the present invention is to create a novel method and device for robotic workcell calibration. The present invention will provide an economical, robust calibration system that will have the ability to calibrate the major considerations involved in any robot system including calibration on a real-time basis during manufacturing processes.
Generally speaking, there are two types of setups in robotic workcells. One consists of the robot holding the tool and workpiece being fixed on the worktable. This is called xe2x80x9cMoving TCPxe2x80x9d. The other type consists of the robot holding the workpiece and the tool is fixed on the floor. This is called xe2x80x9cFixed TCPxe2x80x9d.
In a fixed TCP-based robotic workcell, the forward kinematic chain includes the robot (robot based coordinate), the gripper (work-object coordinate) and the workpiece (object coordinate); the backward kinematic chain includes the tooling system (tool coordinate). In an ideal case, the errors of real or virtual contact points between the tooling and the object are zeros along the working path.
All of the errors from the two kinematic chains can be divided in two parts: xe2x80x9cforward chain errorxe2x80x9d and xe2x80x9cbackward chain errorxe2x80x9d. Forward chain error includes the robot error, the gripper-setup error, and the object-installation error. Backward chain error includes tool-table error and tooling fixture error. The role of calibration is to eliminate or correct all of these errors in order to create highly accurate paths for robot operation. The same principle applies to a moving TCP-based robotic workcell.
In a conventional absolute calibration environment, the goal is to calibrate all the components related to a global absolute reference, in order to eliminate all of these errors separately. Absolute workcell calibration includes robot TCP calibration, tooling calibration and work-object coordinate calibration, wherein each is performed individually. Each calibration process will measure all the Cartesian coordinates to determine the error between the nominal and true value.
Unlike conventional absolute calibration methods, the relative calibration method of the present invention treats all of the errors as relative error between the tooling and the working object compared to a relative reference. Measuring this relative error and finding a way to correct this error is a major advantage of this invention. As long as the relative error is eliminated compared to the relative reference, the workcell is calibrated related to the relative reference and the perfect path will be generated.
Prior to beginning the relative calibration, a computer aided design (CAD) model of the workpiece is downloaded into a data collection and computing device such as a programmable controller or computer.
There are five steps for completing the relative calibration.
The first step is tool center point (TCP) calibration. This consists of performing a TCP calibration using the robot as a measurement tool. The calibration is accomplished by mounting a calibration target within the workcell and in a position that the robot can reach from various orientations. The calibration target can be a sphere, cylinder, cubic or any other definable geometric shape. The robot is programmed to touch the calibration target surface from various angles with a CMM touch probe. All contact positions are recorded. The TCP is calculated from the measurements using a non-linear least squares optimization algorithm.
The second step is to set up a relative reference between the robot and a sample-working object. The relative reference is established by having the robot hold a finished sample of the working object (workpiece) while a series of measurements is performed to compensate for the error between the perfect CAD model and the finished sample to obtain a relative reference. When the actual implementation of the relative method is considered, the enforcement of TCP to follow the standard geometry becomes the biggest concern since the achievable accuracy of a xe2x80x9cstandard referencexe2x80x9d can be very high with moderate manufacturing cost. This compensation process will make the standard reference in a cost-effective way. The actual TCP path becomes the equivalent reference when the enforcement error is treated as the reference geometric error. As a result, the relative reference is the perfect CAD model of the workpiece superposed by all of the system errors from the robot.
In the third step, the robot will hold a raw or unfinished workpiece and the measurement of the raw workpiece will generate a relative error map compared with relative reference set up in the second step.
In the fourth step, an error compensation matrix to calibrate the work-object coordinate, called virtual work-object coordinate, will be calculated based on the relative error map obtained in the third step. An iterative nonlinear optimization algorithm is employed to obtain this error compensation matrix.
In the last step, tooling system calibration will be performed. In the previous steps, the tooling system has not been involved because the calibration station simulates the role of the tooling system. After finishing the calibration of the virtual work-object coordinate in the calibration station, the robot is moved to touch the tooling system and obtain the residual error between the tooling system and the calibration sensor in the calibration station. Compensation of this error into the fixed TCP will complete the tooling system calibration so that the overall workcell calibration process will be completed.
These five steps complete the workcell calibration offline. The subsequent online calibration requires only two steps from those five steps. The first is taking relative measurements of a workpiece utilizing the robot and calibration station. The second step is calculating a new, updated error compensation matrix for the virtual work-object coordinate. These two steps can be performed in real-time and in process.
It will be observed that the fundamental difference between the prior-art absolute calibration processes and the present invention is that the present invention separates the manipulator error and non-manipulator errors by utilizing a relative reference and relative compensation matrix. All of the manipulator errors are assigned to a relative reference while the rest of the non-manipulator errors are dealt with by the virtual work-object coordinate in this process.
It is an object of this invention is to develop a relative calibration method and apparatus to perform in-process calibration on the working factory floor.
It is an object of this invention to provide a method to separate the manipulator error from the non-manipulator installation errors. In this way, a straightforward linear calibration can be implemented in process.
It is an object of this invention to provide a relative reference for relative error measurement. By using this relative reference, a cost-effective high precision measurement tool can be easily utilized.
It is another object of the invention to provide a relative measurement system to measure relative error.
It is another object of this invention to provide an algorithm to calculate the parameters of a virtual work-object coordinate matrix.
Finally, it is an object of the invention to provide a means for in-process implementation of the relative calibration.
In order to implement this method, the corresponding relative calibration system must be integrated in the robotic workcell. The relative calibration system used in this invention includes a robotic workcell including a robot having an end effector. The end effector further includes a gripper for gripping the workpiece. A new substation is added to the conventional robotic workcell, called a calibration station. A sensor for measuring the relative error between a workpiece and the relative reference is located in the calibration station. Within the calibration station there is also at least one calibration sample or calibration target. In addition, a finished workpiece known to be within allowable tolerance requirements is located within the station. A data collection device for recording the output of the sensor and performing calculations is also integrated into the workcell.
The sensor can be a non-contact or contact type linear displacement gauge. A preferred sensor used in the system is a non-contact optical displacement tool. The sensor consists of a diode laser pointer and a linear charge couple device (CCD).
A contact mechanical measurement device could be a linear variable differential transformer (LVDT). The LVDT is basically a series of inductors in a hollow cylindrical shaft and a solid cylindrical core. The LVDT produces an electrical output proportional to the position of the core.